Wednesday, January 30, 2008

Repair Guide: Shock Absorber and Strut Operation

A shock absorber, or strut, is basically a hydraulic damping mechanism for controlling spring vibrations. It will not support weight or return to its original position after it is moved; only a spring will do that. The car body is therefore suspended on springs and the shock absorber is used to control its movements when the springs are deflected by bumps and caused to vibrate.

Shock absorbers control spring movements in both directions: when the spring is compressed and when it is extended. The amount of resistance needed in each direction is determined by the type of vehicle, the type of suspension, the location of the shock absorber in the suspension system, and the position in which it is mounted.

Shock absorbers control suspension vibrations by absorbing the energy stored in the spring when it is compressed and converting that energy into heat. This controls the spring reaction and allows that spring to return to its original position slowly and without a rapid or violent movement. The shock absorber dispels the heat from the converted energy into the air passing around it.

Shock absorbers develop control or resistance.

A shock absorber, or strut, is basically a hydraulic damping mechanism for controlling spring vibrations. It will not support weight or return to its original position after it is moved; only a spring will do that. The car body is therefore suspended on springs and the shock absorber is used to control its movements when the springs are deflected by bumps and caused to vibrate.

Shock absorbers control spring movements in both directions: when the spring is compressed and when it is extended. The amount of resistance needed in each direction is determined by the type of vehicle, the type of suspension, the location of the shock absorber in the suspension system, and the position in which it is mounted.

Shock absorbers control suspension vibrations by absorbing the energy stored in the spring when it is compressed and converting that energy into heat. This controls the spring reaction and allows that spring to return to its original position slowly and without a rapid or violent movement. The shock absorber dispels the heat from the converted energy into the air passing around it.

Shock absorbers develop control or resistance by forcing fluid through restricted passages. There are usually four shock absorbers on a car. One is located near each wheel. They are called direct acting because of their direct connection between the car frame (body) and the axle (wheel-mounting member). Shock absorbers are also called double acting because they control motion in both directions of the suspension travel. Upward movements of the body are called rebound and downward movements are called compression.

The upper shock mounting is attached to a piston rod. The piston rod is attached to a piston and rebound valve assembly. A rebound chamber is located above the piston and a compression chamber below the piston. These chambers are full of hydraulic fluid. A compression intake valve is positioned in the bottom of the cylinder and connected, hydraulically, to a reserve chamber also full of hydraulic fluid. The lower mounting is attached to the cylinder tube in which the piston operates.

During compression, the movement of the shock absorber causes the piston to move downward with respect to the cylinder tube, transferring fluid from the compression chamber to the rebound chamber. This is accomplished by fluid moving through the outer piston hole and unseating the piston intake valve.

During rebound, the pressure in the compression chamber falls below that of the reserve chamber. As a result, the compression valve will unseat and allow fluid to flow from the reserve chamber into the compression chamber. At the same time, fluid in the rebound chamber will be transferred into the compression chamber through the inner piston holes and the rebound valve.

Gas-filled Shock Absorbers

The rapid movement of the fluid between the chambers during the rebound and compression strokes can cause foaming of the fluid. Foaming is the mixing of free air and the shock fluid. When foaming occurs, the shock develops a lag because the piston is moving through an air pocket that offers up resistance. The foaming results in a decrease of the damping forces and a loss of spring control.

During the movement of the piston rod, the fluid is forced through the valuing of the piston. When the piston rod is moving quickly, the shock absorber oil cannot get through the valuing fast enough, which causes pressure increases in front of the piston and pressure decreases behind the piston. The result is foaming and a loss of shock absorber control. The gas-filled shock absorber is designed to reduce foaming of the oil. The gas-filled shock absorber uses a piston and oil chamber similar to other shock absorbers. The difference is that instead of a double tube with a reserve chamber, a dividing piston separates the oil chamber from the gas chamber. The oil chamber contains a special hydraulic oil, and the gas chamber contains nitrogen at 25 times atmospheric pressure.

When the piston rod is moved into the shock absorber, oil is displaced, as in the double-tube principle. This oil displacement causes the dividing piston to press on the gas chamber, thus reducing it in size. With the return of the piston rod, the gas pressure returns the dividing piston to its starting position. Whenever the oil column is held at a static pressure of approximately 25 times atmospheric pressure, the pressure decreases behind the working piston cannot be high enough for the gas to exit from the oil column. Consequently, the gas-filled shock absorber operates without foaming.

Shock Absorber Ratio

Shock absorber control or resistance is expressed as a ratio, such as 90/10, 70/30, or 50/50. There has been no standard usage of the number sequence in the ratio. Shock absorber technicians commonly express the extension control first. A 90/10 ratio means that 90% of the shock's control is in the compression cycle and 10% of the control is in the extension cycle.

Inspect Suspension Components for Wear

SERVICE TIP: One of the best ways to diagnose a suspension problem is to go for a ride with the owner. Observe what the car is doing. Ask yourself if the problem occurs during braking, steering, or over bumps. Your observations will help you a great deal when you inspect the suspension system.

Anytime you have a car on a hoist to lubricate the suspension components, you should also check the suspension parts for wear. A car with poor steering control, rapid tire wear, noise during stopping and driving over bumps, or poor steering stability may have a suspension, steering, or wheel alignment problem. You should follow a systematic step-by-step procedure to determine the condition of the parts in each of these areas.

To begin your inspection, raise the vehicle on a hoist so you have plenty of room to make an inspection. You can test for loose wheel bearings by grasping the front tire top and bottom and rocking it in and out. Any noticeable looseness is too much. To make sure any looseness detected is in the wheel bearing, check for relative movement between the rotor or brake drum and backing plate. Upper and lower ball joints on most vehicles perform different functions. They are designed differently and require different inspection techniques. The design differs because one pair, called the weight carriers, always supports the front half of the vehicle weight. The other set, called the friction or follower joints, supports no vertical load, but must maintain a firm, quiet connection between the spindle and the control arm. On most American cars with short, long arm suspension, the lower ball joint is the weight carrier. But on many import models with short, long arm suspension, the upper ball joint carries the load.

To apply the proper checking procedure, identify which ball joints are the weight carriers and which are the friction joints. The weight carrier is always mounted on the control arm to which the coil spring or torsion bar is mounted. The friction joint is always mounted on the unloaded control arm. After identifying the ball joints by function, continue the inspection.

Friction ball joints have a heavy coil spring or rubber insert inside that preloads the ball stud and enables the spring to dampen the vibration and road shock of the wheels. Unfortunately, the spring also makes the ball joints difficult to check because the preload keeps the ball joint tight and makes a simple wheel shake test unreliable. The most accurate way to check a friction joint for wear is to disconnect it from the spindle, put two nuts back on the stud to act as a lock, and turn the stud with an inch-pound torque wrench. If you get a reading between 24 and 96 inch-pounds (2-8 newton-meters) for most cars, the ball joint is in satisfactory condition. When the reading is above or below these limits, something is wrong in the socket and the ball joint should be replaced. This checking procedure should be followed when the vehicle has very high mileage, has performed in especially rough service, or appears not to have had proper maintenance.

Weight-carrying ball joints are constantly twisting and turning in response to the steering and up-and-down motion of the front wheels. This wearing motion continues while the two ball joints are loaded with the vehicle weight.

To check a weight-carrying ball joint, first visually check for the general condition of the ball joint and seal. Then check to determine the amount of wear on the inside of the part. Look for broken seals that let contaminants into the part, which shortens the life of the joint. When possible, check the throat area under the seal for cracks or distortion.

The next step is to unload the ball joint and measure the amount of clearance. Lower the car to the ground and position a jack to unload the suspension. For lower weight carriers, place the support under the lower control arm near the wheel.

For upper weight carriers, place the support under the frame crossmember and support the upper control arm in its operating position with a wedge between the body and upper control arm. Check for loose wheel bearings and adjust if necessary. Wheel bearings must be snug to avoid adding their clearance to that of the ball joints.

The most accurate way to measure internal wear is by using a dial indicator that can measure vertical movement right at the ball joint socket. This eliminates the chance of adding other clearances, such as those created by loose wheel bearings, to the ball joint check measurement. The dial indicator is mounted on the lower control arm, near the ball joint to be checked, with locking pliers or a clamp. The dial indicator plunger is then placed against the ball joint steering knuckle (depending on the type of car being checked) so it can read the vertical movement of the parts when the ball joint is unloaded and the wheel is moved through its full vertical range with a pry bar.

Lock the indicator in place by tightening the flexible coupling. Set the dial indicator to zero. Place the pry bar between the tire and ground and pry up on the bottom of the tire. Then, when the wheel is moved vertically through its full range, the reading shown on the dial indicator will equal the amount of internal ball joint clearance. Compare your readings to specifications in the shop service manual to determine if the ball joint should be replaced.

The lower ball joint is the only ball joint on MacPherson strut units. To check it, grasp the lower arm near the ball joint and force the arm up and down to check for looseness, or push and pull on the tire near the ball joint. If there is any movement, the ball joint should be replaced.

Many vehicles use ball joints with built-in wear indicators. Wear indicator-type ball joints must remain loaded to check for wear. The vehicle should be checked with the suspension at curb height. The most common type of wear indicator has a small diameter boss that protrudes from the center of the lower housing. As wear occurs internally, this boss will gradually recede into the housing. When it is flush with the housing, the ball joint should be replaced.

You are ready now to visually check each of the control arm bushing assemblies. Control arm cross shafts and bushings provide the inner hinges for the independent front suspension. They attach the control arms to the vehicle frame in a way that permits the wheels to move up and down independently. The control arm bushings provide the bearing for this hinge. The bushings must be in satisfactory condition to perform their primary function of keeping the control arms in the proper position to maintain alignment settings. The rubber torsion type also has an important secondary function of helping dampen road shock from the vehicle body.

Visually inspect each bushing assembly. The first sign of failure in the rubber torsion-type bushing is when cracks appear around the bushing edges. Small cracks on the outer surface are not harmful, but the bushing should be checked very closely when they are present. Look for severe compression of the rubber on one side or the rubber extruding out of the bushing. Also, check for torn rubber and frayed edges. When you examine control arm bushings, also look carefully at the end nuts. They can work loose and allow the bushing to pop out of the mounting.

Coil springs become weakened through constant twisting and flexing in normal service. This allows them to sag, lowering the vehicle out of its normal curb height range. When the curb height varies by even a fraction of an inch from the original specifications, there are problems. Suspension parts, such as control arms and ball joints, may be extremely overloaded when there is not enough suspension travel.

Coil springs are checked by taking measurements at specific points on the vehicle. The measuring points and specifications can be found in the vehicle shop service manual. The springs can also be checked by comparing measurements taken on each side of the car. Measure dimension A; this is the distance between the lower control arm and the frame. There should be a difference of no more than 1/4 inch (6 mm) between the right and left side of the car. Measure dimensions B and C (from ground level to the center of pivot points). The difference between B and C on either side should not be more than 3/4 inch (19 mm). Make sure the bumper is not bent or deformed; then measure distance D, which is the height from a level floor to the bottom of the bumper. The difference between the D measurements should not be more than 3/8 inch (10 mm). If you find your measurements are beyond specifications, both coil springs should be replaced.

Many vehicles are equipped with strut rods that are bolted to the lower control arm and mounted at the opposite end through the strut rod bushing in the vehicle frame. Strut rods act as a brace for the lower control arm. The strut rod bushing must maintain a firm, flexible shock-absorbing mount.

The strut rod bushings wear due to the constant flexing of the strut rod and rubber deterioration caused by the elements. The results are changeable alignment settings, noise (especially during braking), and pulling to the side during braking.

Strut rod bushings wear from the inside out. A check of the external bushing condition will not help determine if the bushing needs replacing. The best way to check strut rod bushings is to raise the car on a hoist with a helper inside. Spin one of the front wheels and then have the person in the car apply the brakes. Worn strut rod bushings will make a popping noise when the brakes are applied. This noise is due to movement in the bushing assembly as the strut tries to control braking forces on the control arm.

Sway bar bushings anchor the bar to the vehicle frame and the control arm on each side. The purpose of the bar is to reduce body roll and sway. The condition of the bushings will affect the performance of the bar. Check the bar for missing links. Inspect the frame bushings for tightness, distortion, and signs of movement by grabbing the bar with your hand and trying to shake it.

Inspect and Test Shock Absorbers and Struts

Whenever you inspect the suspension system, you should test the shock absorbers or MacPherson struts. The test should be performed with the car on the ground, not when the car is being supported on a jack or hoist.

The way to test the front and rear shocks or MacPherson strut cartridges is by bouncing each corner of the car. Rock the car at each corner and release. If the car bounces more than 1 1/2 times after you have stopped, take a closer look at the shocks or cartridges.

If the car bounces more than it should, raise the car back up on the hoist. Run your hand over the tire tread completely around the tire and from inside to outside. Cupping or unusual wear in any area indicates the shocks may not be holding the tires on the road. Look for broken mounts, damaged bushings, and oil on the shock absorber barrel. Grab the shock and shake firmly. This may reveal damage to a mount or bushing not apparent at first sight. Substantial fluid on the outside of the shock absorber housing indicates a leaking seal. Fluid cannot be replaced and shocks are ineffective without fluid; shock absorber replacement is required. Shocks should always be installed in pairs, and it is often most economical to replace all four. One indicator of a need to replace the MacPherson strut/shock is oil leakage at the piston rod seal. Also conduct a bounce test. During the bounce test, carefully observe the top strut mount. Any noise or movement here can indicate the need for parts replacement.

Remove and Replace Shock Absorbers

When replacing front or rear shocks, first compare the shocks on the car to the replacement units. The old and new shocks should be the same length and same mounting; carefully observe the position and type of mounting of the original shock absorbers.

There are three common styles of shock mountings. The shock absorber can be mounted by a thread formed on the end of the piston shaft. This is called a stem mounting. Stem mounting is common on the top end of a shock mounted in the center of a coil spring. The tab stud or cross pin mount is used on the bottom of many coil spring-mounted shock absorbers. Sleeve mounting may be used on one or both ends of the shock absorber; it is used on some front shocks, but is most common on the rear.

Before removing any shock absorber hardware, spray penetrating fluid on all the threads. Road splash and rust can make nuts very difficult to remove. Some nuts will probably have to be removed with an air impact wrench and impact socket.

To remove front or rear shock absorbers, raise the car on a hoist. Do not allow the front or rear drive axle to hang--make sure it is supported on the hoist.

To remove the typical stem and cross pin-mounted front shock absorber, use an open-end wrench to hold the shock absorber stem. Remove the nut, then remove the upper washer and grommet. Unbolt the cross pin from the lower control arm and pull the shock absorber out through the bottom of the control arm.

Make sure the correct number and type of lower rubber grommets and washers are positioned on the stem. Check the instructions with the new shocks and also compare with the old shock mounting. Push the new shock absorber into position through the hole in the bottom control arm. Install the upper washer and grommet, then install the nut. Use an open-end wrench to hold the stem and torque the nut to specifications. Install the bolts that hold the cross pin and torque them to specifications.

Rear shocks are often mounted with a stud or cross pin at one end and a sleeve mount at the other end. The stud and cross pin are removed and replaced by the procedure described earlier for front units. The sleeve mount is removed and replaced by removing and replacing the bolt and nut that goes through the sleeve into the mounting bracket. Be sure to torque all the attaching bolts to specifications.